In orthogonal frequency division multiplexing (OFDM) based communication systems, a high peak-to-average power ratio (PAPR) may decrease the signal quality or deteriorate the transmitting power efficiency for a transmitter. The reason is that the transmitter usually has non-linear RF front end circuits such as a power amplifier. FIG. 1 is a diagram showing the relationship between the input power Pin and the output power Pout of a power amplifier. When the peak power of the input power Pin reaches the non-linear region NL, the output signal of the power amplifier is distorted, and thus decreasing the transmitted signal quality. If the input power Pin is reduced to a value in the linear region of the power amplifier, the transmitting power efficiency will be deteriorated.
In a multi-carrier communication system, e.g. in the orthogonal frequency-division multiple access (OFDMA) mode in compliance with the IEEE 802.16e standard, the base station (BS) can reserve some sub-carriers (which may also be referred to as tones), and reduce the high PAPR by making use of the reserved sub-carriers. FIG. 2 is a schematic diagram showing a conventional method for reducing the PAPR. The original signal which is to be transmitted is denoted as x(t) in time domain. As can be seen from FIG. 2, the to-be-transmitted original signal x(t) exceeds the threshold TH with two peaks. Conventionally, some peak reduction signals P(t) are generated from the reserved sub-carriers, and are cyclically shifted to the time points ni and nj, respectively, so as to provide two signals Pni(t) and Pnj(t). The signals Pni(t) and Pnj(t) are respectively multiplied by a corresponding phase parameter and a corresponding amplitude parameter, i.e. βi and μi as well as βj and μj, and the summation of the multiplied signals and the signal x(t) is performed to obtain a to-be-transmitted modified signal x′(t) with lower PAPR.
However, in the conventional method, at least two Inverse Fast Fourier Transform (IFFT) circuits are required for generating the signals x(t) and P(t). Moreover, in order for the to-be-transmitted signal x′(t) to keep the low PAPR, the conventional method usually performs a procedure of over-sampling on the signal with a over-sampling rate being larger than 4, and performs the above-mentioned procedure of PAPR reduction thereafter.